The long-term objective of our proposal is to elucidate the signaling pathways by which vascular permeability factor/vascular endothelial growth factor (VPF/VEGF, VEGF-A) promotes blood vessel formation and influences microenvironment. VEGF-A participates in a variety of vascular processes including endothelial cell (EC) proliferation, migration, survival, and differentiation (arterial-venous cell fate specification) through its two tyrosine kinase receptors, VEGFR-1 and VEGFR-2 and non-tyrosine kinase receptors neuropilins (NRPs). During the last two cycles of our funding, we defined several diverse and complex pathways of VEGF-A. These pathways focused mainly on proliferation, migration, and survival. Although there is striking evidence for distinct functional roles of VEGF-A-mediated signaling through VEGF receptors (VEGFRs), it is still unclear how certain VEGF-mediated downstream signal transduction cascades selectively potentiate two important functions: promotes EC differentiation and creates EC hyperpermeability. To understand the unique signaling pathways of VEGF-A, we have proposed two aims. Aim 1 will delineate the roles of VEGFR-2 and NRPs for EC differentiation. We will also define the signaling pathways of VEGF-mediated p53 regulation and its role in EC differentiation. Similarly, the key role of Protein Kinase D (PKD) in VEGF-mediated EC differentiation will be evaluated. Whereas, Aim 2 will define the molecular mechanism of VEGF-induced vascular permeability (VP) in real-time. Recently we have developed a heat-inducible VEGF-A transgenic zebrafish model to study VP in real-time. In this aim, we will evaluate the role of individual VEGFRs and the molecules downstream of the pathways that lead to three distinctly different settings: basal vascular permeability (BVP), acute vascular hyperpermeability (AVH), and chronic vascular hyperpermeability (CVH). The proposed studies will elucidate the signaling pathways by which the known as well as unknown molecules mediate the different types of vascular permeability. Hence, the results of the proposed studies will promote understanding of the molecular mechanisms and pathways of these two important functions of VEGF-A and will impact our knowledge of normal physiological processes such as wound healing as well as pathological conditions, including cancer, diabetic retinopathy, and ischemic conditions leading to heart disease and stroke. Lastly, the proposed studies will expand our understanding of VEGF-A signaling as it relates to other VEGF-responsive cell types including circulating progenitor cells, bone cells, and neuronal progenitors.